The processed, always unfertilized eggs of various aquatic animals have been praized for some time as a delicacy in wealthy society and are increasingly consumed. The not yet naturally laid, immature (in various stages) eggs of fish are referred to as roe. The roe may, theoretically, originate from any female fish (or aquatic animal), whose eggs are not toxic. This also includes the blowfish, fugu, which is highly toxic even with proper preparation. So-called “real caviar” or “Russian caviar” is produced from the roe of the sturgeon. One differentiates here between ossietra, beluga, and sevruga caviar. In addition to wild sturgeon farmed sturgeon have also already been used for obtaining caviar. The roe of lumpfish, cod-like fish, and herring is used for producing caviar substitute (“German caviar”). “Icelandic caviar” is produced from the roe of the capelin. Trout caviar is obtained from trout, and salmon caviar having a reddish color is produced from salmon. Furthermore, roe is also obtained from sea urchins. Finally, the amphibians, such as toads, which reproduce by laying eggs, are also to be included in the aquatic animals cited here. Roe is usually salted, sometimes also smoked. Smoked roe is produced from the roe located in the intact envelope by hot smoking. It typically originates from cod and pollack. The roe of lobster, large crayfish, and other crustaceans, as well as the orange-colored roe of scallops, is referred to as “corail”.
Besides the high pleasure value, caviar also has other valuable properties. Caviar is rich in protein (25 to 30%), with a high proportion of essential amino acids. However, with 16% fat it is not a lean food. Caviar contains the vitamins D, E, B12, and niacin, as well as the minerals iodine and sodium. In addition, it has a high proportion of good cholesterol.
The caviar currently available on the market is the treated (cleaned, salted) roe, which is taken in the immature state from the ovaries of female fish, because here the immature eggs have sufficient stability to further treatment steps. Thus, to obtain caviar of the best quality, until now the eggs have been harvested before they have reached their maximum maturity (ovulation stage).
A most significant problem in caviar production is obtaining the eggs. The problem of killing female animals to obtain caviar (e.g., sturgeon) exists both with wild capture and also in aquaculture (approximately 2500 tons/year worldwide). For centuries it has been a standard method to kill the animals before removing the eggs to obtain caviar and roe from various species of fish. A main argument for killing the female individuals is, for example in sturgeon, that the eggs are harvested in the immature state before reaching the natural ovulation stage, because the eggs which have ripened maximally in the body of the mother animal until ovulation, thus capable of being fertilized, are too soft. These mature eggs would burst upon preservation by salt (sodium chloride) alone and in combination with borax and stick together into an inedible slimy pulp. In the course of ovulation, i.e., the ejection of the egg from the follicles of the ovary, the egg undergoes natural changes (softening of the egg membrane for the imminent fertilization by sperm), which restrict its direct use as caviar. In addition, there are osmotic effects which occur later in the course of salting. Experiments using mildly salted sterlet caviar, which was produced for test purposes from ovulated (mature) and non-ovulated (immature) eggs, showed that the mature eggs burst already upon light contact and deliquesced into a greasy, pasty mass. However, significant taste differences were not detectable.
This practice of killing together with drastic overfishing has resulted in threatening of the wild stocks of the approximately 30 different species of sturgeon by extinction. In Russia, a new method has been developed by which immature roe may be obtained without killing the fish. A type of multiple “cesarean section” is performed on the ovaries under anesthetic, the immature eggs are pressed out, and the incisions are closed again. When these are healed, the fish may be released again or kept in breeding stations for later caviar removal. However, the mortality rate for this very complex method, which stresses the fish very strongly, is still above 40%. Moreover, this procedure is forbidden according to the German animal protection code.
Costly restocking programs have been initiated worldwide. Sturgeons are sexually mature in the natural environment for the first time at 8-12 years, in aquaculture earlier at 2-4 years depending on the type of sturgeon. Various species of sturgeon have been released from aquaculture into the wild within the frame of restoration programs with varying success, to save the stocks threatened by extinction. While the female animal remains alive for cultivation and the eggs are obtained by stripping, the problem of killing the female animals still exists for the above-mentioned reasons for caviar production in aquaculture.
High commercial losses are also to be considered for the cultivation of larvae and fingerlings for aquaculture and for stocking measures, however, because the females display significantly improved reproductive performance with increasing age.
Processing after the harvest is desired for the quality of the caviar. Mild salting makes it more storable. High-quality caviar is salted just enough as required for the limited storability. A caviar identified by “malossol” may have a salt content of at most 2.8 to 4%. The natural taste is largely retained in malossol caviar. In addition to malossol caviar, there is also salted caviar, storable for long time, which is mixed with approximately 10-12% cooking salt. Fresh caviar is very temperature-sensitive and thus difficult to store and serve. The traditional form of packaging is the airtight, internally coated can for only salted, but not heated caviar. Pasteurized caviar made storable by brief heating to 60° C. is sold in screwtop jars and ring-pull cans and is storable unopened for one year.
One problem in the storage of fresh caviar is the growth of existing bacteria, resulting in more rapid product spoilage. However, upon pasteurization the caviar loses graininess and taste due to the heat treatment. A further problem in the storage of caviar and other roe products is the crystallization of tyrosine molecules, which are stored in the eggs as nonessential amino acids. The product becomes unsellable due to this crystallization. Furthermore, a problem also exists in the processing speed of the roe obtained. Up to this point, no more than 10 minutes may pass between the harvesting of the eggs from the female and the decanting of the caviar into cans, otherwise the eggs spoil.
However, it is very desirable during every step of the processing (preservation, pasteurization, inter alia) after the harvesting of the eggs that the harvested eggs survive without bursting, because bursting of the eggs results in an irreversible quality loss and thus uselessness of the product.
JP 60091934A describes a method for producing packed herring roe, in which the harvested herring roe, which was taken in the immature stage from the previously killed herring females, is treated antibacterially using a solution made of hydrogen peroxide and salt after a preservation using salt for roe solidification. The concentration of the hydrogen peroxide used may be reduced by immediate blood removal from the harvested herring roe, which results in a taste improvement of the herring roe. After the hydrogen peroxide-salt treatment, an enzymic treatment using catalase is performed to remove the oxyradical hydrogen peroxide. The immediate removal of the still uncoagulated blood may only occur directly after the killing of the fish or shortly before the thawing of the fish, which was frozen directly after killing. Because the immediate removal on the freshly killed fish may only be performed with great effort, however, the fish are typically first frozen, which in turn results in a loss of quality.
Furthermore, tannic acid is a proven agent in aquaculture in obtaining breeding fish. The fertilized eggs are treated using a tannic acid solution to remove the sticky gel layer. The eggs may thus be disinfected better during the breeding, and fungi and bacteria infestation during the egg incubation may be reduced. This treatment is standard for various species of fish such as sea bass, including sturgeon (Mizuno et al.: “Elimination of adhesiveness in the eggs of shishamo smelt Spirinchus lanceolatus using kaolin treatment to achieve high hatching rate in environment with a high iron concentration”, Aquaculture 242 (2004) pp. 713-726). Tannic acid, a polyphenol, is the commercial form of tannin. Its structure is based on glucose esters of gallic acid. Tannic acid is used as a wood staining agent and is naturally present in oak, walnut, mahogany, and sequoia trees as a fire retardant. It may also be inferred from Mizuno et al. (FIG. 5) that a higher egg pressure up to bursting may be achieved by a treatment using tannic acid. However, the egg membrane toughens like rubber due to the treatment with tannic acid and is thus no longer suitable for consumption.
RU 2 126 218 C1 describes a method for dyeing, in which ovulated roe having a weak natural coloration, in particular also of “lower quality” species of fish than sturgeon, are dyed and salted in two steps at an interval of at least 0.5 hours. In the first step, ⅔ of the pigment is added directly to the roe, and in the second step the remaining component of the pigment is introduced together with a lipid-protein emulsion in a quantity of 5 to 10% in relation to the mass of the roe. The salting is performed before the second dyeing step. It may thus not be excluded in this method that the ovulated roe will burst during the following salting after the addition of the dye in the first step. The lipid-protein emulsion is applied after the salting and is used for stable dyeing and improving the organoleptic features and radiation protection properties of the roe.
RU 2 232 523 C2 describes a method for producing granular caviar from ovulated sturgeon roe. For this purpose, the harvested ovulated eggs are first treated in a hot 1.5-2% aqueous solution of a preservative, to prepare them for a following pasteurization at temperatures of 65-70° C. Apart from the fact that every heating procedure significantly influences the taste of the roe, when ovulated eggs are used, which are known to have a very soft egg membrane, it is not reliably ensured that they will endure the following treatment using preservatives without bursting. Even a small proportion of burst eggs worsens the quality of the caviar significantly, because the burst eggs may only be removed with difficulty. However, the fact that the animals neither have to be killed nor treated by stressful surgery for harvesting is of special advantage if ovulated eggs are used.
According to the above, it is thus currently assumed that full value caviar cannot be produced from mature (ovulated) sturgeon eggs, because naturally matured eggs are too soft as a result of the expected fertilization and burst immediately upon contact with salt or other preservatives. The methods discussed above for obtaining caviar from ovulated roe do not provide satisfactory results.